1. Field of the Invention
The present invention is directed to a roll-formed product and a vehicle bumper using the same.
2. Prior Art
A roll-formed product which is in the form of a steel-made tubular configuration is widely used, in various fields, due to the fact that such a roll-formed product is very low in production cost. Today, there is a social request for producing the roll-formed product from much thinner steel sheet, which will contribute energy-saving. Meeting such a request, forming the roll-formed product can be achieved with the usage of a device which is produced at a low cost, with the result that together with a low cost of the raw material the provision of the roll-formed product at much lower cost becomes possible. However, sometimes employing the thinner steel sheet as the raw material of the roll-formed product may raise a problem in strength.
In the field of an automotive vehicle, roll-forming is used for manufacturing various components. In this field, each component is requested to be lightweight from a view point of improving fuel economy which leads to energy-saving. To comply with such a request, using thinner steel sheet is now on demand. However, each of the automotive vehicle components or parts must have a sufficient strength against an outer force applied thereto such as a collision impact or shock, which compels to use a steel sheet having an enough strength and a considerable thickness.
Typical roll-formed products used in automotive vehicles are vehicle bumpers. The vehicle bumper is secured to frontward and rearward sides of a vehicle body for the protection thereof upon collision. To absorb the shock upon collision, the vehicle bumper is requested to have a sufficient strength, while for improving the fuel economy the vehicle bumper has to be lightweight.
In the foregoing circumstances, Japanese Patent Laid-open Print No.Hei.07(1996)-246894 provides an end-closed tubular shaped vehicle bumper which is formed from a continually fed steel sheet having a tensile strength of not less than 6OKSI (413.7 MPa) and a thickness of not greater than 0.1 inches (2.54 mm).
However, using such a high tensile strength steel sheet brings restrictions with respect to roll-forming and cost when producing the roll -formed product. In detail, as the strength of the high tensile strength steel sheet increases, the formability becomes bad, thereby failing to constitute a designed outer profile of the roll-formed product and causing production problems such as lowering rolling speed and shortening the life of the rolling device. In addition, using the highly tensile steel sheet results in that the vehicle bumper becomes poor in shock absorbing ability.